The most affordable shocks are a standard hydraulic design. These work okay in mild applications, but are susceptible to cavitation, or the mixing of oil and air. This occurs in hydraulic fluid when the shock is worked hard, and over time the shock loses its ability to perform. A monotube shock (such as the Bilstein shown) uses pressurized gas, usually nitrogen, under a floating dividing piston (A). Because of the pressurization, the oil in the shock cannot foam. The other mechanical parts inside a monotube shock are the piston and valve assembly (B) mounted on the end of the rod inside the shock body. The valves in this assembly control the resistance to up-and-down movement. Changing the valving in this assembly is the primary way a shock is tuned for a specific application.